The invention concerns a neutral-color gray photochromic plastic article which remains almost completely color-neutral or gray both when darkening and when fading. The plastic article according to the invention may in particular be used as a neutral-color gray photochromic ophthalmic lens.
Photochromic plastic products, especially ophthalmic lenses have been on the market since the Eighties. The first lenses to find a wider distribution, e.g. the Rodenstock Perfalit Colormatic (since 1986) or the tinted Transitions lens from Transitions Optical Inc. (since 1990) and offered by a number of lens manufacturers contained spiro-oxazines as the photochromic dyes which darkened in blue hues. Upon moderate stimulation, this, combined with the brownish pre-coloration of the lens, produced an approximately gray color. Later products such as the xe2x80x9cgrayxe2x80x9d Transitions Plus lens (since 1992), or the brown lenses Transitions Eurobraun and Hoya Sunbrown (since 1994), or the Rodenstock Glas Perfalit Colormatic neu (since 1995) already contained pyrans as well as spiro-oxazines and/or fulgides. Products currently available commercially such as the lens Transitions III preferably use pyrans, especially naphthopyrans and the larger ring systems derived from these. Of these, the Transitions III products with their refractive index of 1.56 are based on U.S. Pat. No. 5,753,1461.
Various color systems have been developed and defined by which to objectively characterize colors. The most widely known, and most suited to ophthalmic lenses, is the L*a*b* or CIELAB color system (1976), in which the brightness (L*) and the color (a*, b*) can be represented by a point within a sphere. During its darkening or fading process, a photochromic lens may be described by a continuous set of points, i.e. a curve within a three-dimensional space. The principal factor in assessing the transmission color of an ophthalmic lens is the projection of this curve onto a central plane, i.e. only the a* and b* values. The system here is equispaced, i.e. the same color distances in the system correspond to the same color differences.
In this color space, a lens which remains at the zero point with respect to a* and b* during the darkening and lightening cycle, i.e. which moves only along the L* axis, represents the ideal case for a neutral-color lens. It passes theoretically from white (=colorless since a lens, especially an ophthalmic lens, is considered in terms of transmission, not reflection) through all gray tones to black, i.e. to complete opacity. In all cases then, the lens is colorless or gray. This mathematical target value isxe2x80x94as are the end pointsxe2x80x94not absolutely attainable in reality, i.e. small deviations from the zero point in the a*b* plane are unavoidable.
U.S. Pat. No. 5,753,146 relates to compositions comprising at least two photochromic naphthopyran compounds which are free of amino-substituted aryl groups at the carbon atom adjacent to the oxygen atom of the pyran ring. These compositions are intended, when inserted into an appropriate carrier and exposed to ultraviolet solar radiation, to exhibit in the activated state a gray or brown neutral color contained within the a* and b* ranges from +10 to xe2x88x9210 of the CIELAB color system. Measurements of the lenses described as prior art in U.S. Pat. No. 5,753,146 have shown that their color locus in the darkened state is at a* =+8 and b*xcx9c+4. During the fade phase, the values even reach a* =+12 and bxcx9c+14. For the chroma C* which is defined as the root of the sum of the squares of a* and b* and characterizes the deviation from the ideal gray point, values are found in the darkened state measuring C*xcx9c9.0, and in the extreme case measuring up to C*xcx9c18.8. As a result, one can no longer speak in terms of a color-neutral lens; rather, the strong color shift during darkening and fade results in an effect which is characterized as the xe2x80x9cchameleon effect.xe2x80x9d Thus the lenses described in U.S. Pat. No. 5,753,146 as photochromic xe2x80x9cgrayxe2x80x9d lenses, as well as other available lenses of prior art, move a considerable distance during darkening and fade from the zero point. To be sure, based on measures described in U.S. Pat. No. 5,753,146, the color loci of lenses in question could be limited to smaller values. The C* value was halved to 9.26; however almost the entire color curve is found in the green-blue color quadrant. The remaining high chroma value as well as the blue-dominated color curve are thus still far from the ideal of a neutral-color gray lens. For the known lenses of prior art, particularly for those described in U.S. Pat. No. 5,753,146, a pronounced color curve during darkening and fade remain characteristic.
The object of the present invention is thus to produce a color-neutral gray photochromic plastic article which remains almost completely color neutral, i.e. gray, both during darkening as well as fade.
This object is achieved by the embodiments characterized in the claims. Specifically, a color-neutral or gray photochromic plastic article is produced which comprises incorporated into it at least two different photochromic color centers from the class of benzopyrans and higher anellated ring systems derived therefrom, the article being characterized in that during a 15-minute exposure of 50 klux at 23xc2x0 C. according to prEN 8980 and a 15-minute fade in the dark, it passes only through color loci for which the chroma is C* less than 8, preferably C* less than 6, more preferably C* less than 5.
In an especially preferred embodiment of the present invention, the color locus of the plastic article, after a 15-minute exposure of 50 klux at 23xc2x0 C. according to prEN 8980, exhibits a C* value of  less than 5, preferably C* less than 4, more preferably C* less than 3.
In another embodiment of the present invention, the spectral transmission of the article according to the invention is preferably under 25%, more preferably under 20% in the range of 400 to 560 nm in the darkened state after a 15-minute exposure of 50 klux at 23xc2x0 C. according to prEN 8980. The spectral transmission of the article according to the invention is preferably under 50% in the range of 700 nm in the darkened state after a 15-minute exposure of 50 klux at 23xc2x0 C. according to prEN 8980.
An important aspect is the most constant transmission possible in the central range of the visible spectrum. Thus the spectral transmission difference in the range of 415 to 540 nm in the darkened state after a 15-minute exposure of 50 klux at 23xc2x0 C. according to prEN 8980 should, for example, be under 10%, more preferably under 8%.
In another embodiment, the spectral transmission according to Vxcex in the completely deactivated state given a 2 mm material thickness without antiglare measures is above 80%. Photochromic lenses are all-purpose lenses; ideally they should replace both a clear lens as well as a sun-protective lens. The resulting requirement is for the highest transmission possible when not stimulated by light such as e.g. at night. Combined with antiglare measures, the aforementioned value allows for transmission values approaching or at 90%.
The plastic article according to the invention may be employed as an optical element such as a lens for which the refractive value measured at the Na-d line lies between 1.49 and 1.76. In particular, the plastic article according to the invention may be employed as an ophthalmic lens.